As is well known, non-volatile memory devices are characterized in that there is no loss of data stored in their memory cells even when an external power supply is removed. For that reason, such non-volatile memory devices are widely employed in a computer, a mobile communication system, a memory card, and the like.
A flash memory device is widely used for the non-volatile memory devices. The flash memory device mostly employs memory cells having a stacked gate structure. The stacked gate structure includes a tunnel oxide layer, a floating gate, an inter-gate dielectric layer, and a control gate electrode, which are sequentially stacked on a channel region. In order to enhance a reliability and a program efficiency of the flash memory cell, a film quality of the tunnel oxide layer should be improved, and a coupling ratio of the cell increased.
Studies on new non-volatile memory devices have been actively made instead of the flash memory devices. A non-volatile memory device, which employs a material having reversibly-varying resistances by applied electric pulses for a data storage material layer, may be advantageous to high integration unlike a capacitor memory device in which memory characteristics are proportional to the size of a cell area. One such non-volatile memory device is a phase change memory device employing a phase change material layer which is reversibly changed between an amorphous state and a crystalline structure by applied electric pulses.
A resistance random access memory (RRAM) device has also been recently introduced, in which a variable resistive material layer is used as a data storage material layer. The variable resistive material layer is a material layer characterized to show reversible resistance variance in accordance with a polarity and/or an amplitude of an applied electric pulse, and a colossal magnetro-resistive material layer (CMR material layer) having a perovskite structure, for example, PrCaMnO3 (PCMO) layer is normally used. However, it may be difficult to apply the PCMO layer to real products since it is difficult to form the PCMO layer on the overall surface of a wafer with a uniform crystalline structure and furthermore, it can be difficult to pattern the PCMO layer by typical photolithography and etch processes normally used in the semiconductor fabrication processes. In the meantime, a transition metal oxide layer is studied as a replaceable material instead of the PCMO layer because the transition metal oxide layer may be more easily deposited and patterned in the typical semiconductor fabrication processes.
In the case of using a phase change material layer or variable resistive material layer as a data storage material layer, an optimized programming method may be used in accordance with the materials or switching mechanism employed. For example, a method of programming a phase change memory device is discussed in U.S. Pat. No. 6,487,113. Further, a method of programming a RRAM device employing the PCMO layer as a data storage material layer is discussed in U.S. Pat. No. 6,673,691. However, such programming methods may not be employed in the same way in the RRAM device employing the transition metal oxide layer as a data storage material layer.